The present invention relates to a liquid toner which is a dispersion of copolymer resin particles containing or not containing a pigment in an electrically insulating dispersion medium, and also relates to a method of producing the liquid toner. More specifically, the present invention relates to a liquid toner which is suitable for use in electrophotography, electrostatic printing, and electrostatic recording, and to a method of producing the liquid toner.
In general, liquid toners consist essentially of pigment particles which are dispersed in an electrically insulating dispersion medium. In development of an electrostatic latent image in electrophotography or the like, however, since development is carried out by an electric double layer of charge carriers formed at the area of contact between the pigment particle surfaces and the electrically insulating dispersion medium, even if a pigment is selected on the basis of color tone or the like, a developing system is determined by the electric charge owned by the pigment particles themselves. Moreover, the charging characteristics are different for different pigments.
Therefore, the conventional practice is to add a charge control agent to a liquid toner so that the charge control agent adheres to the pigment particle surfaces, thereby adjusting the charging characteristics of the pigment particles in the toner. However, since a charge control agent, e.g. a metallic soap, is soluble in an electrically insulating dispersion medium, e.g. an aliphatic hydrocarbon, the charge control agent is readily desorbed from the pigment particle surfaces. Further, if the ratio of the pigment to the resin in the liquid toner is raised, the development properties of the liquid toner deteriorate with time because of sedimentation, aggregation, etc. of the pigment particles. Accordingly, the pigment proportion cannot be increased as desired.
The present applicant has previously found that a liquid toner which is free from problems such as aggregation of pigment particles, and which has high plate wear resistance, excellent transfer properties and high definition can be obtained by dissolving an olefin resin having a carboxyl group or an ester group in a solvent having high temperature dependence in terms of solubility in the olefin resin, e.g. toluene, under heating to prepare a resin solution, and then cooling the resin solution to precipitate resin particles, followed by, if necessary, replacing the solvent with an electrically insulating liquid which does not dissolve the resin. With regard to the above liquid toner, the present applicant has previously filed an application (Japanese Patent Application Post-Examination Publication No. 05-48468). However, there has been a demand for a liquid toner which is superior in dispersion stability in addition to the above-described various properties.
Further, the present invention relates to an ink composition which comprises a dispersion of copolymer resin particles containing or not containing a pigment in a dispersion medium, and also relates to a method of producing the ink composition. More particularly, the present invention relates to an ink composition which may be used as a gravure ink, an ink for color filter, a paint, etc.
In general, conventional ink compositions which have heretofore been used as gravure inks or the like consist essentially of a dispersion medium, particles of a binder resin and a pigment, which are soluble in the dispersion medium, and a dispersant. The conventional ink compositions suffer, however, from the problems that, as the content of pigment and other particles is increased, it becomes more likely that the pigment will be sedimented because of a density difference in the ink, and that aggregation of the pigment will be caused by electrostatic charge of the pigment. Accordingly, the pigment content must be undesirably set at a low level in order to improve the dispersibility of the pigment.
One approach to increase the pigment concentration while maintaining the dispersibility at a high level may be to increase the binder resin concentration, for example. However, if the binder resin concentration is increased, gelation is likely to occur, resulting in coatability being lost. In addition, even if the dispersant concentration is increased, there is a limit to the effectiveness obtained thereby. Thus, no satisfactory results can be obtained; on the contrary, there will be a rise in viscosity and a reduction in shelf stability.